Very few people need a Hearth Flange! Nearly all fireplace doors
rest directly on a hearthstone. That hearthstone both supports the
weight of the door set and also blocks off any openings in the bottom
edge of the door set facing. Those installations DO NOT need a
hearth flange!A few fireplaces are installed higher up on a wall, sometimes called a picture-frame fireplace. In these installations, usually an area of wall is visible BELOW the door set. There are actually two types of these picture-frame fireplaces. (1) Some still have a miniature hearthstone, that resembles a house window's sill, and that small hearthstone can support the weight of the door set, so no hearth flange is needed here either! (2) Some of these picture-frame fireplaces do NOT have a hearthstone that extends out of the wall at all. THIS is the situation where a hearth flange is needed!
In that situation, there is really nothing for the weight of the door set to rest on. A very simple solution to that is to just attach a piece of angle iron to the wall, to make a tiny shelf for the door set to rest on. Depending on a person's choice, it might be possible to mount such an angle with the vertical leg upward (hidden behind the body of the door) or downward (slightly visible. Since most people seem to choose the latter, generally the hearth flanges supplied from the various door manufacturers have a finish that matches the finish of the door set itself.
If a hearth flange is used (instead of a piece of standard angle iron), it is also matched to the thickness of the door set facing, so the width of the horizontal leg blocks off any openings in the bottom of the door set facing. So, even if you are lying on your back on the floor, you would see a nice appearance. More importantly, there would not be any unseen holes under there for a spark from a fire to ever come out into the room.